1. Field of the Invention
The present invention relates to cathodic protection for the prevention of corrosion, and particularly to a portable cathodic protection current interrupter for providing programmable current interruption to cathodic protection current sources.
2. Description of the Related Art
Cathodic protection (CP) is a technique used to control the corrosion of a metal surface by making it the cathode of an electrochemical cell. A simple method of cathodic protection connects protected metal to a more easily corroded “sacrificial metal” to act as the anode. The sacrificial metal then corrodes instead of the protected metal. For structures such as long pipelines, where such passive galvanic cathodic protection is not adequate, an external DC electrical power source is used to provide sufficient current. In the latter case, impressed current cathodic protection (ICCP) systems are used. These consist of anodes connected to a DC power source, which is often a transformer-rectifier connected to AC power.
For pipelines, anodes are arranged in groundbeds, either distributed or in a deep vertical hole, depending on several design and field condition factors, including current distribution requirements. Cathodic protection transformer-rectifier units are often custom manufactured and equipped with a variety of features, including remote monitoring and control, integral current interrupters and various types of electrical enclosures.
For pipelines buried in soil, the applied current changes the voltage across the metal/soil interface, thereby changing the electrochemical state of the structure so that corrosion is mitigated. The voltage across the metal/soil interface is monitored to determine if adequate protection is being achieved. The measured voltage level is typically referred to as a “pipe-to-soil” potential. Various criteria are used in the industry to determine if the pipe-to-soil potential has been shifted sufficiently to mitigate corrosion. The most common criterion is that the potential difference, while the cathodic protection circuits are switched on, is more negative than −0.85 V vs. a calibrated copper/copper sulfate reference electrode. However, an error can be introduced in the measurement if taken while the cathodic protection circuits are switched on. In order to eliminate this error, all influencing sources of cathodic protection current are switched off simultaneously and the pipe-to-soil potential is measured (typically within one second or less) after switching the current off. Such a test is referred to as an “interrupted survey”.
In order to facilitate an interrupted survey, it is typical to temporarily install portable current interrupters into the cathodic protection circuit for the duration of the test. Interrupters are devices that synchronously cycle the current output of cathodic protection systems between “on” and “off”, allowing an interrupted survey to be carried out. For the results to be valid, it is necessary to interrupt all of the influencing cathodic protection current sources. The interrupter is typically connected to a relay, often of the mechanical or solid state type. On some portable interrupters, this relay forms an integral part of the portable interrupter, all packaged into what is typically a relatively bulky enclosure. One disadvantage of this configuration is that the user is limited to a relay of only one particular type and capacity, and is further limited to only one relay per portable interrupter, thus necessitating the use of multiple portable interrupters at a location where multiple current sources need to be interrupted (e.g., locations where multiple cathodic protection rectifiers are installed in close proximity of each other).
In the above, it is important to note that interruption is performed on cathodic protection current output of both cathodic protection rectifiers and sacrificial galvanic anodes. It is common to also interrupt cathodic protection “coupons” from the system for cathodic protection coupon surveys, where cathodic protection coupons are samples of the same material that the protected structure is made from. These coupons are electrically connected to the structure (e.g., a pipe, tank, etc.). Since the coupons are also connected, they receive the same cathodic protection current from the sources (i.e., the rectifiers and galvanic anodes). By interrupting just the connection of the cathodic protection coupon from the structure, a cathodic protection coupon survey that is representative of the structure may be performed. Thus, it should be understood that cathodic protection current interruption is not only performed on cathodic protection rectifiers, but general cathodic protection systems, including rectifiers, sacrificial galvanic anodes and coupons.
Portable interrupters are typically powered in a number of ways. One common method is to connect the interrupter to available primary AC supply, typically 110V or 220V, present in many cathodic protection rectifiers. The internal electronic circuitry of portable interrupters operates off low voltage DC. Some portable interrupters have AC to DC converters built in for this purpose, while others require an external AC/DC converter and accept a 12V DC power input. Sometimes an AC voltage supply is not available, for example, if the cathodic protection rectifier is powered from solar power or with a thermal generator, or if the cathodic protection system is a sacrificial system. In these situations, it is customary to power the portable interrupter with a battery having sufficient charge capacity to last the duration of a test. In some instances, a test may last for a number of days, or even one week or more, and a battery with substantial charge capacity, such as a rechargeable lead-acid automotive battery having a charge capacity of 40 ampere-hours or more may be required. Such batteries are usually bulky, heavy, and inconvenient to use.
Thus, a portable cathodic protection current interrupter solving the aforementioned problems is desired.